I would not advise anyone via computer ... I post here for the purpose of "friendly" conversation !! LOL

Sounds workable ... perhaps ... given your client understands the situation and I suppose is saving a few dollars by not removing the old pond .. the big issue will be drainage for all that rain you get back east ( and I used to get !! ) .. and enough room for your root zone. Two feet is fairly deep but I'm not a Hosta gardener and keep in mind unexpected future use of the garden may un earth what lurks below ground.

It is plausable that the interface between the rubble and added soil and even between the rubble and your base soil may hamper drainage. Could happen ...

I doubt the old concrete will affect the pH in any significant way given it's age and relative location in the soil. Just make sure you avoid an underground "pond"... that could cause sleepless nights ... and dreams of creatures from the deep.

"I have the problem of moving a LARGE amount of soil into the site thru a building. "

Yeah ... thats a problem ... been there .. had to landscape once via a living room with glass tables and white rugs ... this was in a "Master Planned Community" ... no one figured a backyard gate might be a good idea !! Architects ??

Better figure in a contigency factor on this job ... and those Hosta are going to be quite expensive.

Good Day ...

I'm not a pro so I may be way off bat, but couldn't you just fill in the existing pond with peaty soil and make a marsh/bog/marginal plant display?

I once lived in a house with no back access so I know what it's like to hike several tons of gravel through the house in buckets. Good luck!!

"peaty soil and make a marsh/bog/marginal plant display? "

Cool idea ... maybe even a pond ??

I think your passing the buck Seb LOL .. the kosher way to do the job would be to remove the cement.

Good Day ...